Pregnancy due date and number of pregnancy weeks- I'm confused?

We are trying to have a second baby but I'd like it if I can choose the baby's sign - meaning I'll be trying to conceive on certain months of the year so that I may have a child born under a certain sign. Here is my confusion: our first child was conceived some time between Nov 6 and Nov 12. My due date was then Sept 5th, and our daughter was born on August 27th at 38 weeks gestation. How is this possible? I had a perfect and healthy pregnancy - which lasted beyond "9 months" and further more than 38 weeks, even if one were only to count the weeks by conception to birth. Has this happened to anyone else? By the way, I don't need advice from those who wish to convince me that astrology is bunk or anything else, as I am predominantly curious scientifically with the gestational age and due date confusion.. my question is a) whether this has happened to you and b) why.